This invention relates to a tool holder for detachably holding various tools such as drills and end mills.
FIG. 8 shows a conventional tool holder detachably mounted on the spindle of a machining center. It comprises a tool holding portion 40 for detachably holding a tool, a flange 41 keyed to the spindle 1 such that torque is transmitted from the spindle, and a tapered shank 42 adapted to be inserted in a tapered bore 2 formed in the spindle 1. A pull stud 43 is provided at the tip of the tapered shank 42.
The tool holder is coupled to the spindle 1 by inserting the tapered shank 42 into the bore 2 of the spindle, gripping the pull stud 43 with a clamp (not shown) housed in the spindle, and pulling it in.
There are two types of tool holders, i.e. a single-face bind type in which only a tapered surface contacts the spindle 1, and a two-face bind type. The single-face bind type is a type in which the tapered outer surface of the tapered shank 42 is brought into close contact with the inner wall of the tapered bore 2 as shown in FIG. 8. The two-face bind type is a type in which the end face of the flange 41 is brought into close contact with the end face of the spindle 1 and the tapered outer surface 42a of the tapered shank 42 comes into close contact with the tapered inner wall 2a of the tapered bore 2 as shown in FIG. 9.
Tool holders of either type have the following problems. When the spindle 1 begins to rotate at a high speed around 20000 rpm, the diameter of the bore 2 tends to increase under centrifugal force as shown by chain lines a as shown in FIGS. 8 and 9. Thus, with the single-face bind type shown in FIG. 8, a gap forms between the tapered inner surface 2a of the bore 2 and the tapered outer surface 42a of the tapered shank 42. The gap causes the tool holder to be pulled further into the spindle, so that the tip of the tool T held by the tool holder tends to run out and/or displace axially. This makes accurate machining difficult.
With the two-face bind type shown in FIG. 9, too, a gap forms between the tapered inner surface 2a of the bore 2 and the tapered outer surface 42a of the tapered shank 42, though the tool holder is not pulled into the spindle. This lowers the rigidity of the tool holder, thus making accurate machining impossible.
An object of this invention is to provide a tool holder which can maintain rigidity even if the inner diameter of the shank inserting bore increases when the spindle is revolving at a high speed.